1. Field of the Invention
The present invention relates to a permanent magnet synchronous motor which employs permanent magnets for a rotor.
2. Description of Related Art
Permanent magnet synchronous motors typically include a rotor which is rotated by torque applied to magnetic poles of the rotor by a rotating magnetic field generated by applying an alternating current to a stator. The rotor typically includes a rotor core and two types of elongated planar permanent magnets having different polarities and alternately bonded to the outer peripheral surface of the rotor core (see Japanese Unexamined Patent Publication No. 2000-308286).
When the permanent magnets are bonded to the outer peripheral surface of the rotor core (magnetic body), magnetic attractive forces of the permanent magnets act on the rotor core. Therefore, a jig holding each of the permanent magnets should be gradually moved toward the outer peripheral surface of the rotor core to bond the permanent magnets to the rotor core.
To increase the torque of the permanent magnet synchronous motor, it is necessary to increase the diameter or length of the motor. However, where the motor length is increased, the lengths of the permanent magnets to be bonded to the outer peripheral surface of the rotor core should be increased. Therefore, when the permanent magnets are bonded to the rotor core, the accuracy of the bonding of the permanent magnets to the rotor core is reduced. This may result in uneven rotation of the rotor.
Where it is desired to increase the torque of the permanent magnet synchronous motor but the diameter of the motor is restricted, a plurality of permanent magnet synchronous motors are connected axially to each other. However, the permanent magnet synchronous motors each have rotor-absent-portions at coil ends thereof. With the permanent magnet synchronous motors connected to each other, spaces noncontributory to the increase of the torque are present at junctions between the permanent magnet synchronous motors. Therefore, the size of the entire motor is increased as compared with a case in which the torque is increased by increasing the length of the motor. Further, motor control is more difficult.
It is therefore an object of the present invention to provide a permanent magnet synchronous motor in which permanent magnets bonded to the outer peripheral surface of a rotor core are less liable to be damaged and a rotor is substantially free from uneven rotation even if the motor has an increased length.